The interactions of biologically active pepides with macromolecules will be studied via 13C nuclear magnetic resonance techniques. The neurohypophysial hormones, oxytocin and arginine vasopressin, which have been specifically enriched (via solid phase techniques) with 13C in various atoms will be used in conjunction with purified neurophysins. The latter are a group of proteins which bind the hormones in vivo. Experiments will be done to determine which residues are most crucial for hormone-protein interaction, the effects of pH, the stoichiometry and kinetics of binding, and the role of aggregation of the hormone-protein complex. It will thus be possible to get information on hormone-neurophysin interaction at the atomic level, and thereby gain a better understanding of the storage and release of oxytocin and arginine vasopressin. Valinomycin, a peptide antibiotic which facilitates ion transport across membranes will also be synthesized specifically enriched with 13C. The interaction of valinomycin with natural and synthetic membranes will then be studied via 13C NMR to determine the site of metal ion complexation and the effects of different cations, different membrane composition, and temperature changes. From these experiments, a greater understanding of ion transport across membranes will emerge as well as a greater knowledge of membrane structure and function.